1. Field of the Invention
The invention is concerned with the preparation of gas discharge lamps such as xenon containing lamps which also contain a cathode(or cathodes) and an anode therein. Typically the cathode of such lamps is of a refractory metal such as tungsten. Connection between the internal cathode and anode of the lamp and the exterior of the lamp generally occurs via a glass-to-metal sealing of an extension of each of the anode and cathode which extensions pass through the wall of the lamp. Typically, the extensions of the anode and of the cathode are made of molybdenum, tungsten or of another metal having a thermal expansion differential generally within about 1000 parts per million of the lamp wall. Such gas discharge lamps generally operate at a relatively high temperature thus requiring the use of a relatively high temperature glass.
2. Prior Art
The production of high temperature gas discharge lamps with above atmospheric gas pressure therewithin has generally consisted of sealing via quartz-to-metal seals the anode and the cathode of the lamps within the hollow vitreous body thereof. The lamps have generally had a tube extending from an orifice which goes through the wall of the hollow vitreous body, which tube has been attached to a manifold which has been in turn attached via glass stopcocks, metal valves or other valving means to a vacuum pump to allow for complete evacuation from the lamp of all gases therein. Once the lamp has been sufficiently evacuated, the vacuum pump has been shut off from the manifold, and the manifold and thus the lamp have been then generally filled with a measured pressure of the gas to be used to fill the lamp, e.g., xenon gas. The xenon gas in a known volume, which will generally include the volume of the manifold, has then been frozen down within the lamp to form liquid or solid xenon. Alternately a measured volume of liquid xenon is frozen down within the lamps. This has generally been accomplished by bringing a liquid nitrogen containing dewar flask into position about the lamp whereby the liquid nitrogen therein contacts the lamp and causes it to be brought to a temperature well below the freezing temperature of the xenon. After the xenon has been frozen into the hollow vitreous body of the lamp, the tube which proceeds therefrom has then been generally sealed using conventional torch sealing techniques. A serious problem with such a procedure has been premature freezing of the xenon into the tube which is to be sealed off. This has led to clogging of the tube.
When the lamp body is of the usual quartz construction, the tube extending therefrom is usually of the same material and hence the sealing off of the tube generally requires sealing it to a relatively high temperature via a so-called "bright seal" thus requiring much experience and skill on the part of the operator sealing off the tube. Also, the formation of the position of joinder of the tube to the lamp adjacent the orifice which enters the lamp requires working with the relatively high temperature quartz and requires great skill and "bright seal" techniques. Further, because of the very high temperature gradient between the portion of the lamp which is in contact with the liquid nitrogen and the portion of the lamp which is sealed off at a temperature above the softening temperature of quartz, serious thermal strains can result in the structure thus limiting its effective lifetime at the high operating conditions of xenon gas discharge operation. Annealing of the lamp to some extent tends to overcome this problem but it has been found that in practice annealing alone does not completely eliminate thermal strain problems introduced by such a sealing procedure. Very importantly, when operating with a technique as described above it has proven difficult to assure that precisely the desired amount of xenon will be trapped within each lamp despite careful measurement of the volumes and pressures involved. Filling to outside these narrow pressure limits leads to a significant rate of rejection of lamps because such xenon lamps are extremely sensitive to small variations of lamp fill pressure. Overfilling adversely affects lamp life, whereas underfilling adversely affects luminosity. Yet further, the above procedure is quite time consuming requiring carefully made relatively high temperature "bright seals" manually prepared by skilled workers for each individual lamp being prepared and has generally required the use of quartz manifolds or graded glass-to-quartz sealing and/or a metal vacuum system and metal-to-quartz graded sealing and the like in the vacuum system used to fill the lamps thus increasing both the cost of the materials of the manifold and the time required to put it together.
The present invention provides solutions to each of the above set out problems.